1. Field of the Invention
The invention relates to a method and a device for coding at low data-rate cost for conditional-replenishment television systems.
2. Description of the Prior Art
In its simplest form, coding of each sample of a luminance and/or color difference television signal is carried out over a length of eight bits, thus entailing the need for a very high bit rate.
In order to reduce the bit rate while maintaining acceptable picture quality, there are a number of known methods and devices, most of which utilize the redundancy of information contained in an image (picture) or in an image sequence.
One known method consists, for example, after transmission of an initialization image, in transmitting for the following images only those points in which the luminance changes to an appreciable extent from one image to the next. This method is sometimes designated as conditional replenishment and makes it possible to obtain high compression ratios which increase as the detection thresholds (with respect to which the amplitude of the luminance signals is compared) become higher.
As the compression ratios are higher, however, so the image reconstructed at the receiver exhibits a greater number of imperfections which give the viewer the impression of looking through a dirty window.
These defects appear mainly in the pseudo-uniform areas of the image in which the inter-image difference of each point is located in the vicinity of the detection threshold. In view of the fact that the areas stated as moving (moving areas) are replenished and that the areas stated as fixed (fixed areas) are not replenished, this results in fairly considerable coding errors of the order of magnitude of the detection threshold in the case of the points classed in the areas stated as fixed. Since the eye is very sensitive to defects in uniform areas, these errors are all the more visible. A further disadvantage lies in the fact that the position of the erroneous points changes continuously from one image to the next.